1. Field of the Invention
The present invention relates to a process for the manufacture of flexible tubes and more particularly, to that of rubber flexible tubes reinforced with a helical coil of hard or rigid resin material, for instance a hard synthetic resin material. Such reinforced flexible tubes have relatively high pressure resistance, abrasion resistance, chemical resistance, bending strength, tensile strength and the like and thus have been widely employed in various fields to feed air or the like gases, oil or the like liquid, or powder, granule or the like flowable solid substance.
2. Prior Art
Various processes have been proposed for manufacturing flexible tube reinforced with a hard helical coil of, for instance, a hard synthetic resin material (for instance, see U.S. Pat. No. 4,350,547 as well as Jap. Unexamined Pat. Appln. Gazette Nos. 118460/1978, 59746/1982, 38045/1984 and 42943/1984).
In conventional reinforced flexible tubes, there are two types; one wherein a reinforcing helical resin element is fully embedded in or covered with a rubber matrix as a main body or wall of the flexible tube and one wherein a part of the resin element is exposed out of the rubber matrix. At any rate, such conventional flexible tubes have been manufactured by separately extruding a resin strip and an unvulcanized rubber strip, adjoining the resin strip to the rubber strip to form a composite strip or web, feeding the web to a tube former and helically winding thereon, so that a lateral end portion of the web in each turn overlaps on that in the preceding turn to form a tubular body. Then the rubber portion in the tubular body is vulcanized.
According to conventional common techniques, a lateral side end of the rubber strip in the composite web is helically wound and converted into the tubular body. In this case, the overlapped portion is pressed to attain a preliminary bonding at its boundary area and a final bonding is attained through the subsequent vulcanizing treatment.
In other words, the flexible tube based on such common techniques has been manufactured by utilizing a so-called "rubber to rubber" bond of the composite web. Therefore, it is impossible rather than difficult to attain the desired bonding power or strength at the boundary layer and consequently, there is a tendency that strength of the flexible tube per se tends to be reduced, namely in bending strength, tensile strength, pressure resistance and others.
The following may be considered as grounds for causing said disadvantages in the conventional flexible tube of this kind.
(a) Scorching of the rubber element:
The composite web has been formed by extruding the rubber strip and resin strip and making contact of these strips, while in a plastic condition. In this case, when a hard resin material, for instance, a hard polyvinyl chloride is selected for the resin strip, a relatively high temperature of about 170.degree. to 180.degree. C. is required for its extrusion, and the rubber strip contacting to such a high temperature resin strip tends to cause deterioration due to scorching thereof, which will inhibit a development of sufficient bonding power when mutual bonding of rubber portions in the composite web is conducted at a subsequent step.
(b) Shrinkage of rubber element:
The rubber material extruded from a die in the extruder therefor tends to shrink due to its cooling by air. Therefore the resulting rubber strip has an irregular surface which becomes one of causes for reducing the bonding power.
(c) Effect of volatile substances in rubber composition:
The vulcanization for manufacturing the flexible tube of this kind has been carried out under atmospheric pressure. Therefore volatile substances in the rubber strip of the composite web are vaporized and the vaporized substances tend to concentrate at an interface of the rubber surfaces to be bonded to cause a swelling therein or peeling off in the bonded area.
Further, the conventional manufacturing process has a disadvantage in that the tubular body just after its formation by the helical winding of the composite web has a quite low self-supporting ability since the rubber portion has not yet been vulcanized. This requires a careful handling in its transfer during the manufacturing step until its vulcanization has completely been finished, a reduction in feeding speed or the making of the vulcanizing chamber longer and thus those requirements productivity problems and installation space problems for the manufacturing apparatus.